The US operates a constellation of three SDS (Satellite Data Sytem) vehicles in highly eccentric orbits - approx 1,000 x 39,000 kilometres, at 63 degrees inclination. They are in addition to three similar satellites in geosynchronous orbit over the equator.

In each case, perigee is near southern apex so the satellite spend most of its time above the northern hemisphere. It moves slowly so it can be tracked for several hours

The orbit planes of the three satellites are equally spaced around the equator and the satellites are timed to cross the equator at eight hour intervals.

The result is that the follow each other, in procession on roughly the same ground track, also at eight hour intervals.

At the beginning of December 2007, the group consisted of the satellites USA-125 (an SDS-2 vehicle), and USA-137 and USA-179 (both SDS-3 satellites). USA-125 was ageing and was no longer providing the full range of services as earlier in the year. There is also evidence from radio tracking that there was probably a problem with the de-spin mechanism of the aerial array.

The SDS-2 satellies were built by the one-time Hughes Space Systems Division (now part of Boeing). The specialism of Hughes was spin-stabilised satellites with a de-spun platform. There are many examples in orbit performing civilian and military communications roles.

USA-198, a new SDS-3 satellite was launched from Cape Canaveral 2007 December 10. As of early January, it was in an orbit near to its operational one and being prepared to take over from USA-125. Radio tracking indicates that USA-198 took over USA-125's duties during the second half of March.

After handover, USA-125's orbital ground track started to drift westward but it's S-band transmitter continued to operate. Indications from Doppler tracking are that it's orbit-adjustment engine is fired periodically. This could be for testing purposes or there might be an element of wanting to control the drift rate to keep the track near the former operational location. That way it can act as an in-orbit spare.